The monitor mixing engineer is a key function in a public music performance when in-ear monitoring (IEM) is utilized. This paper aims to expand our knowledge about monitor mixing in general and IEM mixing in particular by investigating monitor mixing engineers’ reasoning, decisions and actions. Four experienced monitor mixing engineers were interviewed on monitor mixing in general, IEM mixing and hearing health. Among the results are found that the engineers seek to create a fruitful working relationship with the performers. The engineers also describe what creates a functional mix and they show a high awareness of their responsibility for the comfort and well-being of the artist, both in a psychological sense as well as in providing sound levels that are not harmful.